In the production of fluorosilicone elastomers from low molecular weight, silanol end-stopped fluids, the typical process involves the acid or base catalyzed condensation of the silanol groups to form long chain polymers. Such a process works very well to form high molecular weight gums, i.e. polymers with a viscosity of 1,000,000 centipoise at 25.degree. C. or greater. However, when attempting to form polymers with a viscosity of less than 1,000,000 centipoise (at 25.degree. C.), the final viscosity is very hard to control. As a result, ancillary steps to the process must be accomplished to yield a saleable product.
For example, in the condensation of such polymers by an acid or base catalyst it is necessary to neutralize the catalyst when the polymer viscosity (and hence molecular weight) has been achieved. This neutralization step is necessary in order to prevent the continuance of the reaction. Thus, in a base catalyzed reaction, it is necessary to add enough acid to neutralize the catalyst. Conversely, in an acid catalyzed reaction, it is necessary to add enough base to neutralize the acid. However, if too much acid or base is added to neutralize the catalyst, the neutralizing agent itself will act to further polymerize the material.
In such systems the viscosity of the resultant polymer is hard to control since the viscosity (and hence molecular weight) is dependent upon the amounts of catalysts and neutralizing agents used, the cycle times for the reactions, the presence or absence of polymerization promoters, and the successful removal of catalysts when it is desirable to stop the reaction. In such systems it is difficult to control the polymer viscosity to within 200 centipoise, and it is often the case that the polymerization will run away causing the material in the reaction vessel to achieve a molecular weight which is too high. Consequently, the fluid is not suitable for blending.
It is desirable, however, that the viscosity of the resultant polymer be controlled to within 100 centipoise, more preferably to within 50 centipoise, of the targeted viscosity. To accomplish this it is necessary to develop a new catalyst system to be able to control to greater accuracy the final product viscosity. Thus, it is an object of this invention to develop a process for the polymerization of low molecular weight silicone fluids wherein the final product viscosity may be easily and accurately controlled.